In a typical gasoline-powered automobile engine, a carburetor mixes controlled quantities of filtered air and fuel and feeds the resultant mixture to an intake manifold, from which it is distributed to the cylinder for combustion. The by-products of this combustion are then vented, through emission control devices, to the atmosphere. Engine designers and manufactures have long been aware that as the fuel-air mixture is compressed by the piston in a particular cylinder, a small quantity of the mixture slips by the piston sealing rings, escaping into the crankcase, or interior portion, of the engine block.
Until the early 1960's these blow-by vapors and other contaminents trapped in the crankcase, such as oil vapors emitted by heated engine-lubricating oil, were simply vented into the atmosphere through small ports on the engine block. Since that time, however, engines have been equipped with positive crankcase ventilation (PCV). In this system, a stream of fresh air is directed into the engine interior wherein it circulates, picking up the vapors therein. The contaminated flowing air then leaves the engine through a PCV valve and is conducted by conduit means to the intake manifold, wherein it mixes with the fuel-air mixture provided by the carburetor and is distributed to the cylinders for combustion. Non-combustible components are released, through emission control devices, to the atmosphere. Vacuum in the intake manifold maintains the flow of air through the system.